Steam generator pipes, usually welded to each other in a gas-tight manner via stays for forming a gas draught surrounding the firing chamber are used in the combustion chamber walls of a continuous steam generator, with said pipes being connected in parallel for the throughflow of a flow medium. Instead of pipes with separate flat bar stays lying between them, pipes can also be used which have already been equipped ex-works with fins formed onto them. The steam generator pipes can in such cases be arranged vertically or also inclined. For a safe operating behavior of the continuous steam generator the steam generator pipes are as a rule designed such that, even with low mass flow densities of the medium flowing through the steam generator pipes, a sufficient cooling down of the steam generator pipes is guaranteed.
The heat transfer properties are an important design criterion of a steam generator pipe. A high heat transfer makes an especially effective heating of the medium flowing through the steam generator pipe possible with simultaneous reliable cooling of the steam generator pipe. The heat transfer behavior of a steam generator pipe can be adversely affected in conventional steam generators which are operated at sub-critical pressures, by the occurrence of so-called departures from nucleate boiling. In such cases the wall of the pipe is no longer wetted by liquid flow medium—as a rule water—and is thus only inadequately cooled. As a result of drying out too early the strength values of the pipe wall can then be reduced.
To improve their heat transfer behavior steam generator pipes are normally used which, as a result of a molding process (e.g. cold extrusion) have a surface structure or an inner profile on their inner side in the form of spiral wound ribs. The shape of the ribs imparts a swirl to a medium flowing through the steam generator pipe, so that the heavy liquid phase as a result of the action of centrifugal forces collects on the inner wall of the pipe and forms a wetting film of liquid there. This means that even with relative high heat flow densities and low mass flow densities a reliable transfer of heat from the inner wall of the pipe to the flow medium is guaranteed.
A disadvantage of the known steam generator pipes is that they are comparatively expensive to produce as a result of the limited plasticity of the pipe material. With highly heat-resistant steels in particular with a high chrome content the plasticity is greatly restricted. These types of materials have an ever more important role to play nowadays for steam generator pipes, since they—at least in principle—allow a steam generator pipe to be equipped for especially high steam parameters, especially for high fresh steam temperatures, and thus consequently permit high levels of efficiency. The material-related restrictions mean in practice that it is no longer possible, or only possible at great expense, to create internally-ribbed pipes with the desired rib profiles advantageous for flow within the pipe from smooth pipes within the context of a deformation process. In particular sufficiently steep edge angles and sharp-edged transitions in conjunction with large rib heights can only be produced with difficulty or not at all. In addition the height of the ribs can only be produced within a narrow frame. In addition only a small degree of flexibility is afforded in relation to the design of the profile along the pipe.
As alternatives, different types of swirl-generating fitted parts have already been proposed for retrofitting to a steam generator pipe. These especially include what are referred to as “twisted tapes”: Tapes produced from a metal strip which are twisted or wound together. However the one aspect common to all the pipe inserts known to date is that on the one hand they block up the (originally) free cross-section in the center of the pipe and thus lead to very high pressure loss, and that on the other hand they impart a pronounced redirection to the entire flow and thereby in some cases “overswirl” it. A simple twisted tape for example leads with higher steam content in the two-phase flow to a collection of the water phase in the gusset between the pipe wall and the tape with simultaneous drying out and thereby inadequate cooling down of the inner wall areas on the lee side of the tape, where the lee side refers to the side of the tape that is the steam averted side in the direction of the steam flow. Steam generator pipes with inserts of the twisted tape type are thus not equally suited to all operating conditions usually occurring with steam generators.